Stephen Crocker and Vinton Cerf were among the graduate students who joined UCLA professor Len Kleinrock in an engineering lab on Sept. 2, 1969, as bits of meaningless test data flowed silently between the two computers. By January, three other "nodes" joined the fledgling network.

Then came e-mail a few years later, a core communications protocol called TCP/IP in the late 70s, the domain name system in the 80s and the World Wide Web -- now the second most popular application behind e-mail -- in 1990. The Internet expanded beyond its initial military and educational domain into businesses and homes around the world.

Today, Crocker continues work on the Internet, designing better tools for collaboration. And as security chairman for the Internet's key oversight body, he is trying to defend the core addressing system from outside threats, including an attempt last year by a private search engine to grab Web surfers who mistype addresses.

He acknowledges the Internet he helped build is far from finished, and changes are in store to meet growing demands for multimedia. Network providers now make only "best efforts" at delivering data packets, and Crocker said better guarantees are needed to prevent the skips and stutters now common with video.

Cerf, now at MCI Inc., said he wished he could have designed the Internet with security built-in. Microsoft Corp., Yahoo Inc. and America Online Inc., among others, are currently trying to retrofit the network so e-mail senders can be authenticated -- a way to cut down on junk messages sent using spoofed addresses.

Among Cerf's other projects: a next-generation numbering system called IPv6 to accommodate the ever-growing armies of Internet-ready wireless devices, game consoles, even dog collars. Working with NASA, Cerf is also trying to extend the network into outer space to better communicate with spacecraft.

But many features being developed today wouldn't have been possible at birth given the slower computing speeds and narrower Internet pipes, or bandwidth, Cerf said.

"With the tools we had then, we did as much as we could reasonably have done," he said.